1. Field of the Invention
The invention generally relates to extraction of products from microorganisms and more specifically to methods for the isolation of oils from algae in aqueous media using pressurized carbon dioxide as a solute.
2. Background Information
The biofuels industry represents an exciting opportunity to take advantage of the ability of microorganisms, and in particular algal cultures, to convert carbon dioxide and sunlight to liquid fuels including biodiesel precursors. In this way, microorganisms, especially algae, represent a potentially transformative approach towards renewable liquid fuels less harmful to the environment than fossil fuels. Indeed, microorganisms can offer the potential to capture the CO2 generated by coal fired power plants, ethanol plants, petroleum refineries, and a number of other man-made and natural processes. In order to be successful in making liquid biofuels, such as biodiesel, from microorganisms, the process must become economically competitive in comparison to other alternatives including petroleum, corn-based and cellulose-based ethanol, and other advanced biofuels.
New and effective methods for extracting oils from microalgae are an important area of research and development to make algae a economically viable feedstock for lipid production. In particular, current technologies used for oil extraction require toxic organic solvents as well as expensive centrifuge steps representing nearly 50% of the total capital costs excluding systems to separate the lipid fraction from water and algal biomass. As such, the costs of the lipid purification currently represents perhaps the single most significant cost and technological barrier to algal biofuels commercialization.
Many of the current fuels and manufacturing plants have significant CO2 generation that can be applied both as a feed for algal growth and as a solute to facilitate oil purification. However, the commercialization of algae biofuels will require overcoming a number of significant technological barriers. Indeed, the Department of Energy (DOE) specifically notes methods for extracting oils from microalgae as goal of research and development into algae as a source of feedstock for lipid production.
Significant barriers to implementing algal biofuels and to reaching large-scale algal biofuel production recognized by the DOE, and other federal agencies, include the ability to harvest algae and subsequently remove, or extract, and purify the useful oil from the algal cells that can then be converted to biodiesel precursors. The generation of oil-based biofuels from microorganisms requires that the oil be separated or isolated from the other components in a post growth processing system. It may be desirable to selectively purify neutral lipids as these are the most useful for conversion to a biofuel.
It is widely known that the most expensive processing steps in an algal system are harvesting by centrifugation and lipid extraction. These processes represent a significant fraction of capital costs for algal oil production. Not surprisingly, these problems and the need for solutions were noted in the 1998 National Renewable Energy Laboratory report on algal biofuels. The reasons why further research is required in the area of oil extraction are evident from previous process designs. These designs included expensive and environmentally unsound extraction processes as well as expensive centrifuge steps representing nearly 50% of the total capital costs (excluding overhead costs) to separate the lipid fraction from water and algal biomass.
As a result of the capital and operating costs of oil extraction and centrifugation steps, the expense of the oil purification currently represents perhaps the single most significant obstacle to the commercialization of methods for obtaining biofuels isolated from microorganism. These technological barriers, which give rise to the high costs are due to kinetic (viscosity and density) and thermodynamic (solubility) problems associated with the isolation of, from aqueous biomass, the oils produced or secreted by microorganisms.
There are numerous limitations to the current methods for the isolation of oil from microorganisms. Existing methods of oil extraction from algae and other microorganisms rely on toxic chemicals and/or prohibitively expensive mechanical equipment.
The most common and widely used mechanism for lipid extraction is to add a solvent to a liquid or solid state biomass. This will extract the desired lipid components into the newly-added phase. Multiple extractions are usually required in order to achieve quantitative removal of the lipid material. The extraction produces a high volume, dilute solution from which the desired lipid component then needs to be isolated by expensive distillation or other methods. In addition to the cost of these extraction processes, there are major economic and environmental disadvantages in using liquid-liquid extractions. For instance, this technique utilizes organic solvents such as acetone, pentane, or n-hexane typically obtained from petroleum sources. The process generates large volumes of organic solvents, which may be toxic, creating health hazards for workers and a negative environmental impact. The liquid solvents must either be “washed” for re-use or disposed of, and these activities detract from the goal of a sustainable and environmentally-responsible process with minimal life-cycle implications. All of these disadvantages make conventional liquid-liquid extraction processes untenable for extracting lipids from microorganisms in the long term.
Liquid-solid phase extraction using solute adsorbent materials is another alternative but the costs of these materials are likely to be even more expensive and still require some liquid solvents for isolation of the lipid material from the adsorbent.
In order to avoid the use of environmentally unfriendly chemicals in the lipid extraction processes, the use of carbon dioxide (CO2) has been contemplated for extracting lipids from algae and other plant species. This technique offers a number of advantages for lipid extraction: Removal of CO2 from the lipid phase is much easier than any other organic solvents since a reduction of pressure will enable rapid and inexpensive recapture and recycle of CO2. CO2 is a greenhouse gas but the compound is non-toxic and less harmful to the environment than liquid organic solvents. It is non-flammable (unlike organic solvents) and available at high purity. Carbon dioxide solvent can be easily recaptured for subsequent rounds of lipid extraction or fed to the algae bioreactors. CO2 has a polarity comparable to liquid pentane and is, therefore, ideally suited for extracting lipophilic compounds. Indeed, CO2 is non-polar and will attract non-polar lipids and oils away from water phases and biomass phases. CO2 is relatively low in cost and likely to be widely available at the site of many algae based energy plants. Algae plants require CO2 for growth and many are likely to be co-localized with carbon dioxide producers such as coal powered plants or ethanol plants to take advantage of the local supply of CO2.
It is anticipated that the global demand for energy will double within the next 40 years. This leaves a relatively short period of time for a momentous shift in our sources of energy, and particularly a replacement for fossil fuels. Microalgae represent an incredible opportunity to generate a renewable, domestic source of biofuel compatible with our current infrastructure. Unlike terrestrial bioenergy crops, microalgae do not require fertile land or extensive irrigation, can be harvested continuously, and can be used to remediate the CO2 emissions from fossil fuel combustion. One of the most significant technological and economical barriers to reaching large-scale algal biofuel production is harvesting and extracting oil from the algal cells. While the existing methods of oil extraction rely on toxic chemicals and/or prohibitively expensive mechanical equipment, the process proposed here utilizes high-pressure CO2 to release the oils from the cells and promote the efficient separation of wet algal biomass into oils, water, and biomass. The projections for algal biofuel cost reduction due to this technology are significant.
To date, researchers have used supercritical CO2 as a solvent for extracting lipids from algae and other plant species. The principle of supercritical extraction is essentially the same as liquid-liquid extraction except the solvent is supercritical CO2 rather than pentane or n-hexane. A supercritical fluid is a substance in which the pressure and temperature are above the critical point (pressure and temperature above which fluids no longer have liquid or a vapor phase but have properties that are intermediate). CO2 has a relatively low critical pressure (for example, 74 bar, 1073 psia) and temperature (32° C.) in addition to being relatively non-toxic and non-flammable. Furthermore, supercritical fluids can diffuse through materials easily like gases yet dissolve materials like a liquid, a highly useful property for an efficient extraction fluid. Supercritical CO2 solvent has been used for the extraction of range of non-polar solutes including decaffeination of green coffee beans, the extraction of hops for beer production and the generation of oils and pharmaceutical products from plants. Based on its success in plants, supercritical CO2 solvent has been examined for the extraction of oils from algae by researchers in previous publications.
Unfortunately, the use of supercritical CO2 as an extraction solvent has one major limitation: the amount of oil that can be dissolved in supercritical CO2 is in the range of 0.002% to 0.01% volume of lipid solute/volume of CO2 solvent around the critical temperature and pressure. As a result, the volume of supercritical CO2 solvent that is required for extracting oil from algae is multiple orders of magnitude larger than the oil volume itself. This huge increase in volume of processing will make using supercritical solvent economically impractical for commodity products like fuels due to the huge increase in processing volumes that would be required. The impracticality of using supercritical CO2 solvent extraction can be demonstrated for a case study of an algae species with 40% lipid content. Assuming that the preliminary steps have dewatered the algae to the point of 50% algae and 50% water, the overall lipid content in the mixture would be approximately 20% lipid, 50% water, and 30% additional biomass. For a supercritical CO2 solvent extraction designed to process 100 gallons, the amount of lipids would represent approximately 20 gallons of the total mixture. If the solubility of the lipid in the supercritical CO2 is estimated at optimally 0.1% (or fraction=0.001, which is only achieved at very high pressures), then 20 gallons of lipid would require nearly 20,000 gallons of supercritical CO2 for a complete extraction of the lipids from the mixture. In other words, the processing equipment for the supercritical CO2 solvent alone would represent 20,000/(20,100) gallons or 99.5% of the total volume of the extraction solvent and mixture. 100 gallons of algae-water mixture would require a 20,000 gallon processing vessel in order to use supercritical CO2 solvent. The capital costs associated with building vessels to accommodate a supercritical extractor representing a 200-fold expansion in the volume of the equipment relative to the algal biomass would be enormous. If capital costs increase linearly by volume the use of supercritical CO2 solvent would increase the capital costs by at least 200 fold. As a result, supercritical CO2 solvent extraction is likely prohibitively expensive in view of the processing volumes for extraction of low cost biofuels for the foreseeable future.